According to one of known prior arts, for example, as disclosed in Japanese Patent No. 4309909, a head up display (also referred to as “HUD”) system has a mirror for adjusting a display position and an electric motor for changing a reflection angle of the mirror. The electric motor is composed of, for example, a step motor.
As a driving system for the step motor, a full-step driving system and a micro-step driving system are known in the prior art. According to the micro-step driving system, an interval between neighboring iron teeth of a stator core of the step motor, that is an interval between neighboring excitation stable points, is formed as one step angle, and this step angle is divided into multiple micro angles. A rotor of the step motor is rotated for such micro angles. The micro-step driving system is applied to a driving motor for smoothly adjusting the display position of the image on the windshield.
The micro-step driving system can be applied to the step motor for the HUD system, according to which the image is faded out when an ignition key switch for a vehicle is turned off. In the HUD system, when a display position adjusting switch is turned on in order to adjust the display position of the image on the windshield, the rotor of the step motor moves from one excitation stable point to another (neighboring) excitation stable point.
Since the ignition key switch as well as the display position adjusting switch is turned off by a user (a vehicle driver) at any optional timing, the display position adjusting switch may be turned off when the rotor of the step motor is still on a way from one to the other excitation stable point. In such a case, a detent torque (a holding torque) is applied to the rotor of the step motor, and the rotor is attracted to either one of the excitation stable points. As a result, the display position of the image may be temporarily displaced.
In addition, the ignition key switch may be likewise turned off when the rotor of the step motor is on the way from one to the other excitation stable point, and power supply to the step motor is cut off. In such a case, the detent torque is likewise applied to the rotor of the step motor, and the rotor is attracted to either one of the excitation stable points. As a result, the display position of the image may be also temporarily displaced.
Furthermore, micro-step driving system can be applied to the step motor for the HUD system, according to which a quadrant of electrical degree of the step motor (which corresponds to an excitation stable point for the current rotor position) is memorized in a non-volatile memory device. In the HUD system, when the display position adjusting switch is turned on in order to adjust the display position of the image on the windshield, the rotor of the step motor moves from one excitation stable point to the other (neighboring) excitation stable point.
In the same manner to the above case, the display position adjusting switch may be turned off or the ignition key switch may be turned off to cut off the power supply to the step motor, during a period in which the rotor of the step motor is moving from one to the other excitation stable point. In such a case, the detent torque is likewise applied to the rotor of the step motor, and the rotor is attracted to either one of the excitation stable points. As a result, the quadrant of the electrical degree of the step motor, which may be displaced from the quadrant of the electrical degree corresponding to the excitation stable point of the current rotor position, may be memorized in the non-volatile memory device.
The quadrant of the electrical degree of the step motor, which is memorized in the non-volatile memory device, is used for driving the step motor in a next process for adjusting the display position. Therefore, the display position of the image may be displaced at starting the next process for adjusting the display position.